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Abstract

Long-lived marine megavertebrates (e.g. sharks, turtles, mammals, and seabirds) are inherently vulnerable to anthropogenic mortality. Although some mathematical models have been applied successfully to manage these animals, more detailed treatments are often needed to assess potential drivers of population dynamics. In particular, factors such as age-structure, density-dependent feedbacks on reproduction, and demographic stochasticity are important for understanding population trends, but are often difficult to assess. Lemon sharks (Negaprion brevirostris) have a pelagic adult phase that makes them logistically difficult to study. However, juveniles use coastal nursery areas where their densities can be high. Thus, we use a stage-structured, Markov-chain stochastic model to describe lemon shark population dynamics from a 17-year longitudinal dataset at a coastal nursery area at Bimini, Bahamas. We found that the interaction between delayed breeding and demographic stochasticity accounts for 33 to 49% of the variance. Demographic stochasticity contributed all random effects in this model, suggesting that the existence of unmodeled environmental factors may be driving the majority of interannual population fluctuations. In addition, we are able to use our model to estimate the natural mortality rate of older age classes of lemon sharks that are difficult to study. Further, we use our model to examine what effect the length of a time series plays on deciphering ecological patterns. We find that — even with a relatively long time series — our sampling still misses important rare events. Our approach can be used more broadly to infer population dynamics of other large vertebrates in which age structure and demographic stochasticity are important.

Author Comment

This article is currently being prepared for submission at Biology Direct.

Field Study Permissions

The following information was supplied relating to ethical approvals (i.e., approving body and any reference numbers):

M. Braynen and C. Higgs, Directors of the Bahamas Department of Fisheries, for issuing a scientific permit in support of the field research.

The 2014 permit states: This serves to certify that Dr. Samuel H. Gruber of the "University of Miami", RSMAS, Miami, Florida, USA, and the "Bimini Biological Field Station" has been granted permission to conduct marine scientific research... [on] Lemon shark population dynamics as it relates to abundance, survival, and partitioning of Bimini's lagoon

Grant Disclosures

The following grant information was disclosed by the authors:

National Science Foundation (NSF-OCE 97-12793)

Funding

The collection of data used in this study was made possible with financial support from the Bimini Biological Field Station, Earthwatch Institute, National Science Foundation (NSF-OCE 97-12793), PADI Project Aware, Florida Department of Education (FLORIDA 8749703000001), The Guy Harvey Ocean Foundation. We gratefully acknowledge the following corporate support: M. Aiello, President of Davey Marine; W. Bell, President Sundance Boats; Destron-Fearing Corporation, especially S. Casey; E.R.W. was supported in part by funds from the School of Life Sciences' SOLUR program, at Arizona State University. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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